He became treasurer of the MDC and was nominated by the party's head Morgan Tsvangirai for the post of deputy minister of agriculture.
Al Jazeera's Haru Mutasa, reporting from Harare, said: "We have been told by MDC officials that they fear he may be tortured in the police cell where he is in Mutare ... it could indicate or imply that maybe Zanu-PF isn't really as keen as they say they are to work with the MDC."
Trust Maanda, Roy Bennett's lawyer, told Al Jazeera that Bennett was surprised by the arrest.
"The arrest came as a surprise to Bennett, in the sense that he didn't know of any charges that were being presented against him ... so he came back from South Africa unsuspectingly to Zimbabwe.
"He received assurances from the MDC government that he would not be arrested at all," he said.
Police earlier fired live ammunition in the air to disperse MDC supporters who had surrounded the police station in Mutare in the east of Zimbabwe, the MDC said.
While the MDC said Bennett must be immediately released unconditionally and unharmed, it did not threaten any action that could endanger the new unity government.
Tsvangirai was sworn in on Thursday as prime minister as part of a power-sharing deal with Mugabe.
Against the backdrop of Bennett's arrest, Mugabe swore into office 35 members of a new national unity government aimed at ending years of ruinous political and economic crises.
The swearing-in had been held up by more than two hours as the MDC accused Mugabe of trying to bring 22 ministers into cabinet, although their agreement allowed Zanu-PF party only 15 seats.
In the end, Mugabe swore in two extra ministers while the MDC took one more seat than expected.
An MDC splinter group, MDC Mutambara, has three cabinet posts.
Tsvangirai's party blamed in a statement the development on "backstage chaos and confusion" within Zanu-PF.
Mugabe gave some of his party's staunchest hardliners the key posts of defence, home affairs and national security.
Al Jazeera's Haru Mutasa said there was "nothing new" in the cabinet list as Mugabe had sworn in staunch supporters who have served in his government for decades.
"It's still the old guard and if they come back with the same ideals, the same way of thinking, it will be very difficult for the new prime minister to actually work with them," she said.
"How will he put two different theories, two different groups of people together, how will they come out with one united front to help Zimbabwe to actually find a solution to the humanitarian crisis they are facing?
"Tsvangirai has a lot of work to do, a short time to do it in and people are saying it has to happen quickly or there is a danger that some of his supporters might lose patience with him."
Isabella Matambanadzo, Zimbabwe's programme director for the Open Society Initiative for Southern Africa, said: "It's really critical that this government comes together and delivers together.
"We want to know that we can live in a Zimbabwe where we will not be tortured by our own security forces. We also want to live in a Zimbabwe where we have predictability in our economy."
Control of the home affairs ministry, which oversees Zimbabwe's police forces, was one of the sticking points in power-sharing talks between Mugabe and Tsvangirai.
Zanu-PF and the MDC will co-manage the ministry in the new government.
Tsvangirai faces a series of challenges in his role – from tackling runaway inflation which has left millions of people reliant on food aid, to addressing a cholera epidemic that has left 3,400 dead since August.
Hundreds of thousands of Zimbabweans have left the country, while only 20 per cent of children are going to school.